Some active medical devices are designed to exchange data signals with a remote device, commonly referred to as a “programmer.” The remote device is often a separate external device used to verify the configuration of the coupled device, read information recorded by the device, post information to the device, and update the operational parameters of the device. The data exchange between the medical device and the programmer is done via telemetry, by remote transmission of information without galvanic contact therebetween.
For implantable medical devices, existing data transmission via telemetry is often conducted by an inductive coupling between the coils of the implanted device and those of the programmer. However, this inductive coupling technique has a disadvantage, because of the requirement of physical proximity between the implanted device and the programmer and a “telemetry head” connected to the programmer such that an operator places the coils of the programmer in the vicinity of the site where the device is implanted to establish inductive coupling.
Recently, a non-galvanic and non-inductive coupling technique was proposed for data communication via electromagnetic wave between two components operating in a radiofrequency (RF) domain, typically in the frequency range of several hundred Megahertz. This technique, referred to as “RF telemetry” allows programming or interrogating an implanted device at a distance greater than 3 m from a programmer, thus it enables exchange of data without a telemetry head, or even without an intervention by an external operator.
One active medical device employing such an RF telemetry system is described in EP 1862195 A1 and its counterpart U.S. Pat. No. 7,663,451 (both assigned to Sorin CRM S.A.S., previously known as ELA Medical). The communication protocol between an active device (which may be but is not necessarily an implantable device) and its base station (e.g., a programmer or a “home monitor” device) is governed by the industry standard EN 301 839, Electromagnetic Compatibility and Radio Spectrum Matters (ERM)—Short range devices (SRD)—Ultra Low Power Active Medical Implants (ULP-AMI), and Peripherals (ULP-AMI-P), operating in the frequency range of 402-405 MHz. It is noted, however, that the RF telemetry system described herein is not limited to the 402-405 MHz frequency band for Medical Implants Communication Systems (“MICS”), but is generally applicable to any bands that could be used for the RF telemetry, including, for example, the Industrial, Scientific and Medical (ISM) public unmarked bands 863-870 MHz, 902-928 MHz, or 2.4 GHz, as may be used by medical devices.
Moreover, medical devices equipped with RF telemetry functions are generally multichannel devices using multiple frequencies in one band, but some devices may also be multi-band.
Therefore, prior to establishing communication between an active device and its base station (programmer), it is necessary to select a channel (i.e., a communication frequency) among those available channels in the given frequency band. This step is important because it is essential to select with confidence a channel having a noise level such that the communication can be carried to completion without being interrupted.
Unlike induction telemetry techniques that have generally good noise immunity, the RF telemetry technique is subject to numerous electromagnetic disturbances, including radio, television and mobile phone signals, or industrial or environmental noises that may be present in the vicinity of the patient. The device, particularly when communicating on an RF channel, can also be in conflict with other nearby devices trying to connect in the same RF channel. All these disturbances are likely to cause interferences and disrupt the data transmission. If an RF channel that is too noisy is selected, the data communication is subject to transmission errors, leading to the abandonment of the process and the search for a new channel. It may be required to repeat the communication process until a successful communication is established, and consequently it causes energy loss for the unsuccessful communication attempts. RF telemetry involves relatively high energy consumption, at a significant scale for an implantable device whose useful life is critical, so that multiple interrupted communications can pose a significant impact on the autonomy of the device.
When selecting a communication channel, an initial or previous scan is involved to successively “listen” to the various available channels for transmission, before choosing one of them to start broadcasting signals.
U.S. Pat. No. 6,868,288B2 describes a technique for selecting a communication channel by simultaneously analyzing the average received signal level on each channel, and then classifying these channels according to the respective detected signal level. Any signal detected on a given channel is considered as noise. Each selected channel is then analyzed separately to verify that it is available when the device starts to emit signals. However, between the moment of the scan and the start of data transmission, communication may have started on that same channel from another device, so that the selected channel may actually be occupied and no longer available.
This technique of channel selection based on simultaneous scan and subsequent analysis for channel selection based on the signal level has two main drawbacks. First, because of the selected analysis criterion, any signal present in the selected channel is regarded as noise. In other words, the analysis does not distinguish between background noise (i.e., “real” noise) and a valid communication data signal on this channel: the latter is regarded by the device as noise, while this data communication may take place on a quality channel in terms of Signal Noise Ratio (SNR). In other words, the device may exclude such a quality channel, only due to the fact that this channel is occupied at the moment of the scan.
Secondly, during the signal scan, the analysis of an average signal level on a given channel does not guarantee the availability of the channel when the device begins to broadcast data signals on that channel. The risk of an inability to establish successfully data communication remains, and in such cases the established communication channel needs to be switched to another channel by repeating the process, resulting in an increased loss of energy.
It should be understood that this risk is particularly aggravated in a hospital environment where multiple devices are simultaneously present from the same manufacturer using the same channel selection algorithms. For example, multiple devices may try to establish an RF communication channel at moments relatively close in time, and because of this slight temporal shift, a first device may consider a channel is free, while another device starts to emit a little time later and make communication impossible for the first device, forcing it to repeat the channel scan to search for another channel. The presence of a plurality of products of the same origin in the same site may cause unexpected interference with each other, requiring an operation logic with complex escaping loops.
This phenomenon is further enhanced by the relatively high overall occupancy of various channels in a clinical environment, but the problem of finding an available channel is superimposed on the problem of finding a noisy channel.
U.S. Pat. Publication No. US2008/015655 A1 (Bange & Al.) describes a comparable technique but with the same drawbacks; it is impossible to assess the risk of establishing a channel that has a low noise level at a given moment but is not the optimal channel at another moment when it is really to be used, and so throughout its use.